According to IGN Japan, it's definitely a Wii Mario game.
http://wii.ign.com/articles/988/988444p1.html
"For Mario, the article says absolutely nothing -- just that it's a new entry in the popular series. It lists this as being one of Nintendo's tools for pushing domestic (Japanese) Wii sales, so this is definitely a Wii game. "
In which case it would most definitely be Super Mario Galaxy 2, because Iwata or someone said that he wanted to expand on the ideas of the first one.
So the Nintendo conference can focus on Pikmin 3, Wii Fit Plus, and Wii Sports Resort while still mentioning Super Mario Galaxy 2 (no need to go too in-depth here, people know what they're getting) and possibly ending with a Zelda teaser. All this plus some DS stuff and the usual we sell more than the rest PR.
Is that even possible in the two hours(?) of the conference?
# of games above 75 on Metacritic (including downloadable):
360: 241 DS: 144
PS3: 152 PSP: 126
Wii: 85
